The invention relates to a circuit arrangement for signal coupling between circuit parts having supply lines isolated from one another, in particular for use in vehicles containing a data recording unit communicating with other vehicle devices, the data recording unit and the vehicle devices interchanging signals with it being connected using supply lines which are DC-decoupled from one another. Hence, the invention relates, in particular, to a circuit arrangement for signal coupling between ignition-linked and non-ignition-linked units and devices in a vehicle, said circuit arrangement being arranged in a data recording unit.
In a vehicle, data recording units, such as a tachograph, are connected to other vehicle devices, such as a speed sensor, a speed limiter, a combined instrument or similar units and devices for the purpose of data and signal interchange, these units and devices being able to be connected to various supply lines in order to receive their operating voltage. There is thus a continuous positive available in a vehicle, which means to say a direct supply of electrical power from the vehicle battery, or a supply of electrical power routed via fuses or current-limiting elements. Furthermore, there are supply lines which are DC-decoupled from the continuous positive and can be connected using the ignition of the vehicle and/or a main switch.
On account of its intended purpose of permanently recording driver activities and vehicle movements, a tachograph arranged in a vehicle has circuit parts connected to the continuous positive. However, it also communicates with vehicle devices whose operating voltage is connected only by actuation of the ignition and/or of the main switch, this connected operating voltage also being used to supply power to those circuit parts which, despite being arranged in the tachograph, are connected to those vehicle devices from which the tachograph receives the signals to be recorded. Hence, the tachograph contains circuit parts which are connected to different supply lines and between which signals of various nature need to be interchanged in turn in the tachograph. These signals may be static or low-frequency signals, or else dynamic signals with frequencies up into the MHz range. Furthermore, digital signals and data are increasingly also being interchanged between a tachograph and the remainder of the vehicle devices via a databus, such as a CAN bus.
Various problems result from the fact that the data recording unit, for which a tachograph has been selected here by way of example, contains circuit parts which communicate with one another and are connected to different supply lines. Thus, interference may arise during signal transmission on account of different ground potentials resulting from voltage drops along the wiring harness arranged in the vehicle. In addition, significant currents must not flow from the data recording unit to the vehicle devices connected to it, nor should such a current flow from these external devices into the data recording unit, which could occur, however, if the connectable supply lines' associated negative line, i.e. the line for the ground potential, which the voltage present on the respective supply line takes as reference, is disconnected.
To prevent these undesirable effects, optocouplers or high-value series resistors could be inserted into the signal paths, for example. However, these measures have the disadvantage that optocouplers suitable for rapid signal transmission are relatively expensive and that, if high-value series resistors were used, it would be necessary to ensure that the ground potentials of the operating voltages present in the data recording unit differ from one another only to such a small extent that the internal switching thresholds of the signal receivers arranged in the respective circuit parts are not exceeded. It is not possible to guarantee that such a condition is observed in every case under the customary operating conditions for a vehicle, however.
The object of the present invention is to demonstrate a circuit arrangement for signal coupling between circuit parts having supply lines isolated from one another, said circuit arrangement ensuring reliable, interference-free and economical transmission of static and dynamic signals under the aforementioned operating conditions.
The object is achieved by the features of the first claim. The dependent claims illustrate advantageous developments and refinements of the solution found.
The solution found is characterized by the fact that, in connection with capacitive coupling of DC-decoupled circuit parts, the signals to be transmitted are subjected to forced clocking, which also enables static and low-frequency useful signals to be transmitted via the signal paths provided with coupling capacitors. In addition, the proposed solution contains measures for stabilizing the input voltage range of the devices receiving a signal in the circuit arrangement in relation to the output voltage range of the devices transmitting a signal, in order to make reliable, interference-free and economical transmission of static and dynamic signals possible despite the rough environment prevailing in vehicles.